1. Field of the Invention
Embodiments of the invention generally relate to methods and apparatus for use in oil and gas wellbores and, more particularly, to performing sonic well logging within such wellbores.
2. Description of the Related Art
Downhole logging tools may be used to temporarily measure many important downhole properties and conditions in oil and gas wells. These tools may be typically classified in two categories: open hole and cased hole. Open hole tools may be designed to measure rock properties in the formations surrounding non-cased wellbores, as well as the properties of the fluids contained in the rocks. Cased hole tools may be designed to measure fluid properties within a cased borehole and also to examine the condition of wellbore components, such as well tubing or casing. Cased hole tools may, in some cases, also measure rock and fluid properties through the well casing.
Formation properties that may be important in producing or storing fluids in downhole reservoirs comprise pressure, temperature, porosity, permeability, density, mineral content, electrical conductivity, and bed thickness. Fluid properties, such as pressure, temperature, density, viscosity, chemical elements, and the content of oil, water, and/or gas, may also be important measurements. Downhole properties may be measured by a variety of sensing systems in these downhole tools, including acoustic, electrical, mechanical, magnetic, electro-magnetic, strain, nuclear, and optical based devices.
Downhole logging tools based on sonic well logging systems may be used to measure downhole properties such as formation porosity, location of bed boundaries and fluid interfaces, well casing condition, and behind casing cement location and bonding quality.
Permanent or semi-permanent monitoring of downhole properties and conditions may be related to the temporary measurements made with downhole logging tools. However, the measurements may involve the long-term emplacement of the sensing systems. Monitoring downhole formation properties over time may have significant value in better producing, injecting, and storing reservoir fluids. Permanent or semi-permanent in-well sensing systems commercially available today may measure borehole pressure, temperature, flow and phase fraction, vibration, seismic response, and micro-seismicity. Other concepts and prototypes exist for permanently or semi-permanently emplaced formation evaluation tools utilizing technologies such as resistivity, acoustics, electric potential, and nuclear radioactivity.
Sonic well logging systems typically require an acoustic energy source and an array of one or more acoustic sensors or receivers. The sensor arrays may consist of multiple discrete devices, and the deployment of an array of sensors may be complex and expensive. Permanently (or semi-permanently) deployed sensors must be able to withstand the downhole environment for long periods of time. In some cases, the downhole temperatures may be very high.
The deployment of a multi-sensor acoustic array currently requires the use of multiple electrical conductors conveyed from the surface to the downhole sensors, sophisticated downhole electronics, or optically multiplexed sensors. Optically multiplexed sensor arrays have been deployed, based on Bragg gratings, for seismic imaging and monitoring and for sonar acoustic based flowmeters. Similar techniques using optical Bragg gratings may also be used to deploy an array of acoustic sensors for permanent or semi-permanent sonic well logging.